Au is the second strengthening grade of u, the counterpart of ai on the other side of the vowel system, and with it the vowel section of the syllabary closes. The four long vowels that end the series therefore form two matched pairs rather than a list of remainders: e and ai built up from i, o and au built up from u, each pair one step and then two.
In the vocabulary the vowel is not common, but where it occurs it is doing recognisable work. The second grade marks words derived from other words, so a term beginning with this vowel is usually a derivative naming something belonging to, descended from or characteristic of whatever its base names. Reading Sanskrit is in large part the habit of noticing such signals, and this letter is one of the loudest of them.
In metre it counts as heavy, like every long vowel. On the page it follows the ordinary rule: written in full only where a word opens on it, and elsewhere as a mark attached to the preceding consonant. With this letter the reciter of the table crosses from the vowels to the consonants, and the two sections are learned, counted and named as distinct halves of one order.
The independent letter is rare in running text and most readers meet it first in the chart rather than on a page. The dependent mark is another matter, and turns up wherever the second strengthening grade is used, which in a text discussing schools, doctrines and derived qualities is frequently. The two forms of one vowel can have very different frequencies.